INTRODUCTION Industrial and instrumentation applications (I&I) require transmission of data between multiple systems often over very long distances. The RS-485 bus standard is one of the most widely used physical layer bus designs in I&I applica-tions. The key features of RS-485 that make it ideal for use in I&I communications applications are • Long distance links—up to 4000 feet. • Bidirectional communications possible over a single pair of twisted cables. • Differential transmission increases noise immunity and decreases noise emissions. • Multiple drivers and receivers can be connected on the same bus. • Wide common-mode range allows for differences in ground potential between the driver and receiver.